Deposits to the Thrift Savings Plan are made by employees, not retirees, so you cannot contribute anything to the TSP after you have retired.
You are, however, still allowed to rollover/transfer qualified funds into the TSP from qualifying accounts. You could transfer a Roth 401(k) type account into your Roth TSP balance, and a traditional 401(k) account into your traditional TSP balance. You could also transfer and pre-tax money from a traditional IRA to your traditional TSP balance. You cannot transfer a Roth IRA into your Roth TSP.